The present disclosure relates to a foreign object detector, a method of detecting a foreign object, and a non-contact charging system.
In recent years, wireless power transmission systems (i.e., non-contact charging systems) have been, and are being, developed to charge, for example, electric vehicles via a non-contact method. In each wireless power transmission system, a transmitting′ coil and an RF oscillation source are provided for a charger, and a receiving coil is provided for an electric vehicle. Some wireless power transmission systems using an electromagnetic induction method enable high-efficiency non-contact power transmission.
Such a wireless power transmission system using the electromagnetic induction method is designed to transmit a large quantity of electric power. Therefore, if a metallic foreign object enters the gap between its transmitting and receiving coils and/or its environment, there is a risk that the system would generate heat. In view of this consideration, it is thus important, for safety reasons, to detect such a metallic foreign object before or during charging.
As a conventional method of detecting a foreign object in a wireless power transmission system, there is a method of detecting a change in the inductance of a detection coil to be caused by electromagnetic induction between the metal and the detection coil (see, e.g., Japanese Unexamined Patent Publication No. 2012-16125).
If a plurality of detection coils are arranged near the transmitting coil to detect a metallic foreign object on and/or around the transmitting coil, an AC magnetic field generated by the transmitting coil induces a high voltage in the detection coils. This results in breakdown or malfunction of a detection circuit.
An RF oscillation source of a charger is generally configured as an inverter circuit to output a large quantity of electric power. Since the inverter circuit makes switching using a square wave, not only a voltage of the oscillation frequency but also a voltage with a frequency component that is sufficiently higher than the oscillation frequency, such as that of switching noise, are induced in the detection coils. In Japanese Unexamined Patent Publication No. 2012-16125, the frequency utilized in detecting the metal is set to be higher than that of a magnetic field generated by the transmitting coil to reduce the influence on the detection.